The Vendor Shuffle
Yep. I don't do the full Obsidium Shuffle (my enchanter is too low level), I'm a "buy, cut, vendor" type. Since the information about the lowered vendor cost of gut uncommon gems on the PTR came out, I've actually been on one giant last hurrah of vendor shuffling Obsidium. This is not because I make sooo much gold shuffling 48g stacks of Obsidium or whatever, but that because I'm either making small amounts of gold, or, worst case, breaking even, on the uncommon quality gems, this is actually a source of essentially free rare-quality gems. I'm about half stockpiling and half cutting-AHing the rares I get.

Rare gems
Inferno Ruby: I try to sell at least 1 of each main stat (int, str, agi) each day. On my server, the Strength rubies sell the fastest and most frequently.

Ember Topaz and Demonseye: I actually sell these uncut. They are popular enough gems that people usually buy them (if not immediately, then on the second relist), but there are so many possible cuts, and so few sockets that people might use these gems in, that I find I do less relisting selling them uncut than cut. I think about 75% of my AH strategy is based around that I hate relisting.

Ocean Sapphire, Dream Emerald, Amberjewel: TANK GEMS. Oh my god. People keep going on about how these color gems don't sell, and no, they are not nearly as expensive as the red/orange/purple gems, but they do sell. Slowly enough that I try to never have more than one of a cut up at a time, but they sell. Especially now that everyone that can tank is gearing up a tank set so they can take advantage of the Dungeonfinder Call to Arms coming in 4.1. I sell the +Stamina blue cut, the +Stam/+Mastery green cut, and the +Mastery yellow cut. If there's some fierce undercutting going on on the +Mastery Amberjewel cut, I sometimes try selling a +Resilience cut instead. These usually cost more, but require more relisting.

Volatile Life --> Volatile Air
My most dependable income after vendor shuffling. Depending on transmute procs and if I buy my Life or not, this is anywhere from 150g to 400g profit pretty much guaranteed every day. For like, literally 5 minutes of effort.

Low level enchanting mats
I'm pretty much not selling lowbie enchanting scrolls right now. Partially because I'm low on Small Radiant Shards and Wildvine for Minor Haste and Fiery Weapon enchants at the moment, and am a bit nervous about buying them instead of farming my own. But also because the more serious business twink/BoA enchant market is just a little bit too already spoken for for me.

So I've figured, if I can't compete with them on the scrolls, I can at least take their money selling them mats. Plus, I like farming anyway.

I leveled my priest's Blacksmithing from 300-340 straight through with Enchanted Thorium Blades, so I've got a ton of Greater Eternal Essences I'm slowly selling off. Illusion Dust is also fairly pricey, so all greens of around the level to produce these materials get sent to my enchanter right away.

As for stuff I farm (or have farmed in the past), I've also sold some Essence of Air and Essence of Undeath, both of which have gone up in cost fairly dramatically recently. I am definitely thinking of starting to schedule a little time one day each week for farming these materials. Possibly also Wildvine if it's as rare as it seems to be from the few times I've looked for it on the AH.

I also definitely need to get my enchanter to the point when she can solo some of the later vanilla dungeons for enchanting mats. SOME DAY.

I solo TBC instances for rep. Constantly. I have buckets of netherweave and no tailor to make bags. I have a set price I always sell netherweave for (as expensive as I can and still be pretty much guaranteed that the Netherweave bag makers on my server will buy it), and while it's not a huge amount of my income, it adds up.

Outland materials
Primals (except earth), Khorium bars, Fel Iron ore... People buy it.

Prospecting any Adamantite Ore I mine
No, really. On my server I've found this is more profitable than selling the ore most of the time. (However, I made a few Adamantite rods while leveling BS, and I am not sure that that might not be more profitable. I'll need to monitor prices for a while.)

Adamatite Powder is combined with Primal Earth to make Mercurial Adamantite. This moves slowly, but moves, and is the best way I've found to move my Primal Earth aside from bargain basement prices.

Rare quality gems are vendored. They vendor for 3g uncut.

Green quality gem are put on the AH for folks leveling jewelcrafting. I like to wait to post them until they're up to at least 1g75s per gem.

Golden Draenite (the yellow uncommon gem) is a special case. It's used in crafting the flying carpet mount, and therefore sells for much more than the other color gems. When I prospect some Golden Draenite, I do a little dance.

Deepstone Oil
My guild is still working on getting the +90 stat feast recipe, which means I (and the one other fisherman in guild) need to fish from pools a lot. A lot. I decided to go fish up some Albino Cavefish, make some Deepstone Oil, then sell it in 8 stacks for people having the Vial of Sands mount crafted. This is not a hugely profitable venture, since not that many people are having the mount crafted, but they do move, albeit a bit slowly.

Stuff that I've tried and am not super impressed with:
Titansteel. I have buckets of Titanium bars to get rid of. (I transmuted a bunch when I was leveling Alchemy, since it was pretty easy to get the Saronite bars.) So I thought making titansteel when I had the Eternals available might be a good choice. But I'm not totally convinced it's not better to just sell the Eternals and find something else to do with my Titanium bars.

Primal Might. Similar to the Titansteel. With the amount of listing/relisting, I think it would have been better to just sell the primals. the one thing this does have going for it is that it does use up that pesky bankspace eating Primal Earth.

Vendor recipes. The market's just so flooded, it's not worth getting in on. I'm working on getting rid of what I have in my bank, but I'm not adding them to my "to pick up regularly" list. Waste of gold for me.

Stuff I need to try
Selling to the Horde. I have no interest in getting a second account to sell stuff directly to the Horde on their own AH, but I think I may try putting some Alliance pets up on the neutral auction house and see how quickly they sell. I used to do this, but got out of the habit. Now that my mage is high enough level that she can port all over the place, I might set her hearth to Booty Bay, and look into doing this again, on a more scheduled basis.

Inscription. I hate hate hate hate hate relisting. Hate it. This is why I changed my original Scribe over to a Blacksmith and washed my hands of the market. But I think I ought to give it another try.